Equilibrium Studies of Binary and Mixed-Ligand Complexes of Zinc(II) Involving 2-(Aminomethyl)-Benzimidazole and Some Bio-Relevant Ligands

Abstract

Binary and mixed-ligand complexes of zinc(II) involving 2-(aminomethyl)-benzimidazole (AMBI) and amino acids, peptides (HL) or DNA constituents have been investigated. Ternary complexes of amino acids or peptides are formed simultaneously. Amino acids form the complex Zn(AMBI)L, whereas amides form two complex species Zn(AMBI)L and Zn(AMBI)(LH−1). The ternary complexes of zinc(II) with AMBI and DNA are formed in a stepwise process, whereby binding of zinc(II) to AMBI is followed by ligation of the DNA constituents. The stability of ternary complexes is quantitatively compared with their corresponding binary complexes in terms of the parameters Δlog10 K, log10 βstat and log10 X. The effect of the side chains of amino acid ligands (ΔR) on complex formation is discussed. The values of Δlog10 K indicated that the ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids. This may be taken as evidence for a stacking interaction between the aromatic moiety of AMBI and the aromatic side chains of the bio-active ligands. The concentration distributions of various species formed in solution were also evaluated as a function of the pH.